The ionic dependence of the response of hyperpolarizing cells in the scallop retina to a cessation of illumination has been investigated. The off response of these cells is associated with a voltage-dependent transient response which is activated by the depolarization (off receptor potential) that occurs on cessation of lights. This response can be elicited in darkness when external Na ion is removed from the bathing medium and the membrane is hyperpolarized to a value more negative than -60mV. The transient has a threshold, is 30 to 40 mV in amplitude and 250 to 350 msec in duration. Our results show that the transient is caused by an increase in Ca2 ion permeability. The rate of repolarization and amplitude of the after potential that follows the transient are also Ca2 ion dependent. Since both the repolarization and after potential phases are K ion dependent our results suggest that an influx of Ca2 ions during the depolarizing phase of the transient response activates an increase in K ion permeability which assists in membrane repolarization. The transient response can be abolished by agents which block Ca2 ion conductance changes (D-600, Co2 ion, La3 ion, etc.). Our results show that under these conditions the cessation of light produces a simple depolarization which is caused by a decrease in light induced K ion conductance.